1. Field of the Invention
This invention relates to a light shielding cover for a sheet-like light guide member for guiding light, which is emitted from a stimulable phosphor sheet carrying a radiation image stored therein when the stimulable phosphor sheet is exposed to stimulating rays such as laser beams, to a photodetector.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultra-violet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and read-out system. Specifically, the stimulable phosphor formed on a sheet is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as laser beam which causes it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor sheet upon stimulation thereof is photo-electrically detected and converted to an electric image signals which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
Since the light emitted from the stimulable phosphor is non-directional and weak, it is necessary to increase the light guiding efficiency by enlarging the light-receiving solid angle as much as possible so as to catch more light. If the light guiding efficiency is low, the signal-to-noise ratio becomes low and, in the worst case, it becomes impossible to obtain a signal from the light.
As described in U.S. Pat. No. 4,346,295, it has been proposed to position a sheet-like light guide member made of a light-guiding sheet material between a stimulable phosphor sheet and a photodetector for detecting the light emitted from the stimulable phosphor sheet in such a manner that one end face of the light guide member is positioned close to the scanning line on the stimulable phosphor sheet, and the other end, which is formed to match the shape of the light receiving face of the photodetector, is closely contacted with the light receiving face, thereby to increase the light guiding efficiency and improve the signal-to-noise ratio.
The sheet-like light guide member is made of a transparent thermoplastic resin sheet such as an acrylic resin sheet so that the light entering from the light input face can be transmitted to the light output face by total reflection through the interior of the light guide member. The light emitted from the stimulable phosphor sheet when the phosphor sheet is exposed to stimulating rays is guided in the interior of the light guide member, emitted from the light output face of the light guide member and received by the photodetector. Preferred shapes and materials of the light guide member are described in U.S. Pat. No. 4,346,295.
When the sheet-like light guide member as mentioned above is used, it is possible to detect the light emitted from the stimulable phosphor when the stimulable phosphor is exposed to stimulating rays at a very high light guiding efficiency. However, it has been found that light emitted from portions of the stimulable phosphor sheet outside the scanning portion of stimulating rays also enters the sheet-like light guide member and, as a result, the signal-to-noise ratio of the obtained image signal is decreased. The light emitted from portions outside the scanning portion of stimulating rays includes the following:
(1) light emitted from portions of the stimulable phosphor sheet outside the scanning portion when stimulating rays entering the sheet-like light guide member together with the light emitted from the stimulable phosphor sheet leak from the light guide member before the stimulating rays go to the light output face of the light guide member, and excite the aforesaid portions of the stimulable phosphor sheet to emit light, and PA0 (2) light emitted from portions of the stimulable phosphor sheet outside the scanning portion when the stimulating rays reflected from the stimulable phosphor sheet and then reflected from the light input face of the sheet-like light guide member excite the aforesaid portion of the stimulable phosphor sheet to emit light. PA0 (1) light emitted from the stimulable phosphor sheet when the stimulating rays leaking from the sheet-like light guide member of the final read-out system excite the stimulable phosphor sheet in the final read-out to emit light, PA0 (2) light emitted from the stimulable phosphor sheet when the stimulating rays leaking from the sheet-like light guide member of the final read-out system excite the stimulable phosphor sheet in the preliminary read-out to emit light, and PA0 (3) a portion of the light emitted from the stimulable phosphor sheet by scanning thereof with the stimulating rays in the final read-out, which could not be caught by the sheet-like light guide member of the final read-out system.
Further, it has been proposed in commonly assigned U.S. Patent Appln. Ser. No. 434,886 to employ a method wherein, prior to final read-out for obtaining a visible image for viewing purposes, preliminary read-out for detecting the recording information stored in the stimulable phosphor sheet is conducted by use of stimulating rays having an intensity lower than the intensity of stimulating rays used in the final read-out, and final read-out conditions are determined based on the recording information obtained thereby. In general, this method is conducted by installing two light detecting means respectively comprising the sheet-like light guide members in a single read-out apparatus. In this case, since the sensitivity of the preliminary read-out system is generally several tens of times higher than the sensitivity of the final read-out system, the signal-to-noise ratio of the signal obtained in the preliminary read-out is markedly decreased if the light emitted from the stimulable phosphor sheet upon stimulation with the stimulating rays in the final read-out system enters the sheet-like light guide member of the preliminary read-out system. The light emitted from the stimulable phosphor sheet upon stimulation with the stimulating rays in the final read-out and directed into the sheet-like light guide member of the preliminary read-out system includes the following:
Therefore, it is desired to shield the sheet-like light guide member from the light emitted from portions of the stimulable phosphor sheet outside the scanning portion of stimulating rays. However, when a light-shielding plate is used for this purpose, it constitutes an obstacle to the loading of the sheet-like light guide member on an apparatus and the unloading thereof from the apparatus. Further, if the sheet-like light guide member is coated with black paint, it becomes impossible for the light guide member to transmit light by total reflection. Accordingly, it is considered desirable to employ a light-shielding member covering the sheet-like light guide member in spaced relation thereto. However, as disclosed in U.S. Pat. No. 4,346,295, the sheet-like light guide member is made by heating and softening a transparent thermoplastic resin sheet such as an acrylic sheet while an end portion thereof is maintained in the flat form, rounding the other end portion of the sheet to a ring-like shape and clamping the portion, allowing the sheet to cool naturally, cutting both end portions of the sheet, and polishing the cut faces. Therefore, the shapes of the sheet-like light guide members thus made are not always the same, and fluctuation occurs particularly in the shape of the side face (as will be described later) and the linearity of the portion from the light input face to the light output face. Since the shapes of the respective sheet-like light guide members differ from one another, it is not always possible to shield the light guide members from light by use of moldings having a predetermined shape and made of plastics, metals, or the like. Particularly, it is difficult to shield the portion near the light input face from light. Further, the cost required for manufacture becomes very high.